The invention relates in general to a method and an apparatus for the control of a positioning drive and, in particular, to such control in an elevator installation.
The typical positioning drive has a cascade structure in which the biasing of an appropriate jerk pattern and a threefold integration over time of the same generates the desired distance value S.sub.S, as well as the desired velocity and acceleration values V.sub.S and B.sub.S respectively. When utilized with an electric motor, the velocity and acceleration values are applied directly to the velocity and armature current control circuits respectively for the control of the object to be positioned. Such controls improve the dynamic behavior of the positioning drive so that the actual travel curve better follows the specified optimum desired travel curve. The drive can then be brought up to speed optimally, that is under control, and the best possible utilization of the conditions defined by the desired travel curves can be made in order to reach a predetermined position.
Every positioning drive used in a control system must move to any desired position while maintaining specified conditions. Sometimes, the conditions require that the tolerance ranges for the positioning accuracy and the running-in velocity for the destination position are very narrow, or that the destination position must not be overshot. Frequently however, the positioning process has to be concluded in the minimum possible time, where limit values for jerk, acceleration, deceleration and velocity specific to the installation have to be maintained. Furthermore, there may be a requirement for minimum lost energy. In all these cases, however, the major factor in determining the accuracy and speed of positioning is the regulating or control device and the desired travel curve acting on it as a command variable.
A method and a device for the control of a positioning drive are shown in the German Pat. application 3 001 788, wherein a variable command generator generates the desired travel curve which acts on a cascade control. In the command generator, command values are formed for the desired position value by a threefold integration over time of the predetermined jerk values. The acceleration, that is the integral over time of the jerk, is generated by a starting controller which is limited to the maximum jerk. The desired value of the acceleration is varied at small displacement distances dependent on the remaining distance and at larger displacement distances dependent on the velocity. The desired values generated for distance, velocity, and acceleration are entered as bias values to the cascade control, where the desired values of velocity and acceleration are input directly to the velocity and armature current controllers respectively.
Since, according to the above described method of control, the desired value of acceleration at short displacement distances is dependent on the remaining distance, the problem of the precise determination of the remaining distance is present. The remaining distance is determined not only at the beginning of each short displacement distance, but also is determined continuously as the difference between the actual position of the destination and the desired distance value as determined by the command generator. This determination of the remaining distance assumes, therefore, that the actual value of the distance follows the occasional changes of the desired distance value with minimal lag error. If this is not assured, the generated travel curves will not be optimal, due to the inaccuracy inherent in them, so that the end portion of the travel distance has to be travelled at a creeping velocity in order that generated control mistakes can be equalized. In order to form an optimal travel curve, a good response behavior of the cascade control is essential.
In the case that the optimum desired travel curves, calculated from imputed data and provided destinations by known travel curve computers, are available, there results an optimal travel only if the actual value of distance is able to follow the desired value of distance at all times. Thus, the control device must exhibit a minimum distance control error. It has been found that subordinated velocity and armature current control circuits, as well as their forward correction by appropriate velocity and acceleration values, as shown in German patent appliction 3 001 718, are often insufficient to guarantee the accuracy of guidance which is necessary in high-grade positioning installations which require, for example, high stopping accuracy. This is particularly due to the frequently important load changes which, from travel to travel, can act as disturbances in a positioning installation. Thus, a further drawback is created in that such regulated drives frequently have to be oversized in order to be able to precisely follow the desired distance value even in the most unfavorable case of loading. Obviously, the economy of such devices is thereby impaired. The present invention provides a remedy for such problems and deficiencies.